Firefox offers HTTPS Mode, which converts all HTTP links to HTTPS first, A HREFs, stuff you type into the URL bar, everything, then if that fails it generates an interstitial page explaining what went wrong with a button to get the unencrypted HTTP site if that's available.

But that's optional (I wouldn't recommend it to anybody who doesn't seem clear on what HTTPS versus HTTP means for example) and far more invasive, though in exchange it delivers more practical security if you understand what's going on. I've enabled it, I have mentioned it to IT people I know socially, I wouldn't suggest my mother or sister try it.